1. Field of the Invention
The invention relates generally to the optical processing of line patterns, of multiple line patterns or of grey scale data that must be processed for pattern recognition purposes and, more particularly, to automatic optical surveying of such patterns for the generation of binary coded data patterns.
2. Description of the Prior Art
The prior art includes optical systems for the analysis or recognition of fingerprint patterns that employ matched optical filtering or cross-correlation techniques for the direct comparison of the image of a fingerprint to be identified with those of prerecorded fingerprint images. In addition, related techniques have enabled comparison between the Fourier transform of the image of a fingerprint to be identified and the prerecorded Fourier transforms of fingerprints. Such optical systems have often involved rapidly rotated slit or grating optical filter elements generally disposed in the Fourier transform plane of an optical processor for cyclically selecting distinct components of the Fourier transform for transfer to an image plane for detection there and subsequent processing. Data representing ridge line angular orientation is generally developed, though the spacing between ridges may also be derived. Representative concepts of this type of recognition apparatus appear in the following United States patents assigned to Sperry Rand Corporation in the name of the present inventor:
U.s. pat. No. 3,771,124--"Coherent Optical Processor Fingerprint Identification Apparatus", issued Nov. 6, 1973, PA1 U.s. pat. No. 3,771,129--"Optical Processor Fingerprint Identification System", issued Nov. 6, 1973, PA1 U.s. pat. No. 3,873,970--"Fingerprint Identification Apparatus", issued Mar. 25, 1975 (with W. T. Maloney), PA1 U.s. pat. No. 3,882,462--"Fingerprint Recognition Apparatus Using Non-Coherent Optical Processing", issued May 6, 1975, PA1 U.s. pat. No. 3,891,968--"Coherent Optical Processor Apparatus With Improved Fourier Transform Plane Spatial Filter", issued June 24, 1975, and PA1 U.s. pat. No. 3,968,476--"Spurious Signal Removal in Optical Processor Identification Apparatus", issued July 6, 1976.
However, there is a need in certain applications for fingerprint recognition devices having adequate performance but employing reliable relatively inexpensive optical components, thus avoiding rapidly rotating parts to perform specialized filtering functions, which parts in themselves, are delicate and expensive to manufacture and which present problems of proper alignment along an optical axis. While such problems are readily solved as in the systems of the foregoing patents, the present concept is desirable in certain circumstances because it simplifies the optical hardware and represents an approach characterized by a reduction in the number of production problems that the manufacturer must face.
A prior art concept provides a synchronous overlapping raster scan of two colors. One color is used to form a finely focussed spot on the scanned image. The size of this small spot is comparable to or small compared to the image details to be resolved. The second color is used to form a larger or more coarsely focussed spot that is concentric with the small spot and of a size that is large compared to the image details to be resolved. Light scattered from each of the two spots are separated by filters and then directed to different light level detectors. If a white card is illuminated by the two colors at the same radiant power, the light levels detected by each detector will be equal. On the other hand, if there is a fine grating-like pattern on the card and the small spot instantaneously resides on a line of the grating that is darker than the surrounding area average of grey values, the light scattered from the finely focussed light beam will be less than that scattered from the larger area light beam. The difference in scattered light power received by the two detectors may then be used to classify that point on the pattern as black.
Numerous problems are inherent in the two-color concept which can be cured only by using more complex and costly hardware implementations. The light power of both beams must accurately track through lapses of time, the sensitivities of the detectors must track for varying signal strengths, and the light received by both detectors must track as the image is scanned. These and other characteristics must be controlled or calibration is lost. The purpose of this invention is to provide a relatively simple optical reader system that accomplishes essentially the same results as the more complicated prior art devices.